Aluminum foils are popularly used in heat exchangers because aluminum has very high thermal conductivity. These fins are typically fitted over copper tubes and mechanically assembled. As the size of the air conditioner units increases, the fins become longer, and it is important that they have sufficient strength so that they can be lifted without bending. Low strength can also result in handling damage when the coils are bent to form a unit. One way to improve the rigidity of the coil is to increase the gauge of the aluminum foil. Since this alternative is costly, and adds weight, air conditioner manufacturers prefer to use stronger foil.
The most popular alloy used in this application is the alloy AA 1100. It has the composition shown in Table I below:
TABLE IElementsWtSilicon + Iron:<0.95Copper:0.05–0.20Aluminum:>99.00Other elements:<0.05
When fully annealed, this alloy has very low strength. For example, typical yield strength could be between 20.7–41.4 MPa (3–6 ksi), and ultimate tensile strength (UTS) could be between 96.5–110.3 MPa (14–16 ksi). This alloy is highly formable, with elongation generally exceeding 24% and Olsen values above 0.25 in. (6 mm). If the formability is inadequate, the collars formed in this sheet through which the copper tubes are passed can crack in the reflare or in the body of the collar itself. These cracks are undesirable because the copper tubes, after passing through the fins, are expanded to form a good joint between the collar and the tube. If the collar is cracked, heat transfer between the fin and the tube deteriorates. “0” temper, AA 1100 sheet forms excellent collars and is popularly used in this application. A problem arises when higher strength is desired in applications such as long fins.
Typically, AA 1100 alloy formed by direct casting or DC method, hot rolled and then cold rolled to the final gauge of 0.1–0.13 mm (0.004–0.005 in), can be partially annealed. The partial anneal step involves heating the cold rolled sheet at temperatures between 240–270° C. During this time, the strength of the cold rolled sheet decreases and its formability increases. The cold rolling destroys the aluminum structure completely. When it is heated, the first step involves recovery and the second step involves recrystallization. In a typical anneal, the step of recovery involves a gradual reduction in strength while recrystallization involves precipitous decline in strength. The typical desired mechanical properties of a partially annealed sheet are shown in Table II below:
TABLE IIYield strength (MPa)96.5–110.3Elongation (%)20–24 UTS (MPa)110.3–124.1 
The partially annealed material has a structure that is fully recovered and has started forming some initial grains (incipient recrystallization). These grains are small, typically less than 25 micron in diameter. This material performs extremely well in fin application with collar cracks generally below 5%.
DC casting method, however, is expensive. In recent years, there has been a trend to go to continuous casting, using belt casters, roll casters, or other similar equipment. Continuous casters produce an “as-cast” strip that is less than 30 mm in thickness (more generally less than 25 mm in thickness). Roll casters generally produce a strip of 6 mm or less that can be directly cold rolled. Belt casters produce strip that can be either directly cold rolled or may be used in conjunction with an in-line rolling mill that reduces the thickness of the as cast slab, after it is solidified but before it cools, to a thickness suitable for cold rolling. The hot rolling step in DC cast material is preceded by a preheat (homogenization) at around 500° C. This homogenization step is not present in continuous casting method, and thus the thermal history of the two materials is significantly different. As a result, DC cast AA 1100 material produces excellent partially annealed sheet, whereas the corresponding continuous caster (CC) cast sheet has so far failed to give the desired performance. CC cast material is less formable than DC cast material at equivalent strength. Attempts to improve the formability (as characterized by elongation and Olsen values) by increasing the anneal temperature results in reduction of yield strength significantly below the lower limit of 89.6–96.5 MPa.
Various studies and previous attempts have been made to develop improved methods of making aluminum foils utilizing a single roll continuous casting method and an aluminum based alloy composition which can be single roll cast, homogenized, cold rolled and annealed to produce an aluminum foil product. For example, U.S. Pat. No. 5,466,312 (Ward, Jr.) discusses a method of making an aluminum foil which comprises providing a molten aluminum-based alloy consisting essentially of about 0.08 to 0.20 weight percent silicon, about 0.24 to 0.50 weight percent iron, and about 0.21 to 0.30 weight percent copper, with the balance being aluminum and inevitable impurities. The aluminum alloy composition is continuously cast to form a coiled cast strip. The coiled cast strip is homogenized, cold rolled, and followed by a final recrystallizing annealing step of 450–650° F. This temperature range creates recrystallization in the foil.
U.S. Pat. No. 5,554,234 (Takeuchi) proposes high strength aluminum alloy suitable for use in the manufacture of a fin. According to the patent, the aluminum alloy contains at most 0.1% by weight of silicon, 0.10 to 1.0% by weight of iron, 0.1 to 0.50% by weight of manganese, 0.01 to 0.15% by weight of titanium, with the balance being aluminum and unavoidable impurities. The patent also discusses a method of manufacturing a high strength aluminum alloy suitable for use in the manufacture of a fin, which comprises the step of heating an aluminum alloy ingot to 430–580° C., hot rolling the ingot to obtain a plate material, and applying a homogenizing annealing treatment at 250–350° C. for the stated purpose of causing intermetallic compounds to be distributed within the metal texture of the alloy.
U.S. Pat. No. 4,737,198 (Shabel) discloses a method of casting an alloy having components in the composition range of about 0.5–1.2% iron, 0.7–1.3% manganese, and 0–0.5% silicon by weight, homogenizing the cast alloy at temperatures below about 1100° F., preferably below about 1050° F. to control the microstructure, and cold rolling to a final gauge. The cold rolled alloy is then partially annealed to attain desired levels of strength and formability.
Japanese Patent No. 5-51710 proposes an aluminum foil annealed at 150–250° C. in a hot air furnace which carries the foil along on a hot air cushion at a temperature of 350–450° C. Japanese Patent No. 6-93397 discusses an aluminum alloy for making a foil and a treatment method to improve the properties of the foil, including cold rolling, heat treatment up to 400 C., and then process annealing at 250–450 C., followed by further cold rolling.
It is an object of the present invention to provide an improved method for producing aluminum alloy foil for heat exchanger fins based on continuous casting of an AA 1100 aluminum alloy.